1. Field of the Invention
The present invention relates to a method for cleaning polysilicon (polycrystalline silicon) to serve as a material for the manufacture of silicon single crystals.
2. Prior Art
Silicon single crystals for semiconductor devices have been principally manufactured by the Czochralski method (to be referred to as Cz method hereinafter). The Cz method consists of melting solid or granular polysilicon in a quartz crucible in a furnace, immersing a crystal seed into the melt thus obtained, raising the crystal seed thereby allowing single silicon crystals to grow. Because polysilicon to serve as a material for the manufacture of single silicon crystals has a high reactivity, it is stored being helmetically sealed in a plastic bag just until it is transferred into a quartz crucible. However, when polysilicon is placed in a bag for sealing, or removed from a bag for use, it often reacts with oxygen in the air to form thereby an oxide film on its surface. The oxide film may have organic materials, particles or metal impurities adsorbed on its surface. If polysilicon with such organic materials or the like adsorbed on its surface is melt, the growth of single silicon crystals is disturbed so that the freeing rate (the weight ratio of a single silicon crystal obtained from a unit weight of polysilicon) is problematically reduced.
To serve as a solution to the above problem, a method has been known in which polysilicon is cleaned with a mixed acid comprising hydrofluoric acid and nitric acid just before the polysilicon is transferred into a crucible. With this method, however, organic materials and others dissolving in the mixed acid may easily re-adsorb on the surface of polysilicon. Thus, with this method it has been difficult to remove organic materials. Moreover, the method requires such a large quantity of the mixed acid that the cost for cleaning is increased.